A51 may refer to: Area 51, the nickname for a military base in Nevada that is the subject of many conspiracy theories A51 Terrain Park (Colorado), a terrain park in Keystone, Colorado A51 road, a road in England connecting Kingsbury and Chester A51 autoroute, a road in France connecting Marseille and Grenoble A5/1, in cryptography, a stream cipher used in GSM cellular networks Samsung Galaxy A51, a smartphone released in 2019 Budapest Gambit in chess (Encyclopaedia of Chess Openings code "A51", among others) A-51, a Namibian hip hop band